[CodeGenCXX] do not default to dllimport storage for mingw-w64

[martell]

Summary

GNU frontends don't have options like /MT, /MD etc so it makes little sense.
This fixes a few link error regressions with libc++ and libc++abi

[rnk]

Saleem, didn't something change here two months ago? Why do we want to bother with all of this dllimport stuff in the frontend? It seems to break mingw and add complexity for very little gain (avoiding linker provided thunks).

[mstorsjo]

Does this break things for current users of clang as a drop-in replacement for gcc in mingw setups, while using e.g. a gcc provided libstdc++-6.dll? OTOH, removing dllimport usually isn't too much of an issue - the calls go via thunks which is non-ideal and theoretically slower but not much of an issue in practice.

[mstorsjo]

After trying to build a bit more code with clang and a static libcxx, I agree with this change, so +1 from me. @compnerd ?

cfe/trunk/lib/CodeGen/CodeGenModule.cpp

llvm::Constant *C =
                               /*DontDefer=*/false, /*IsThunk=*/false,
                               ExtraAttrs);
 
   if (auto *F = dyn_cast<llvm::Function>(C)) {
     if (F->empty()) {
       F->setCallingConv(getRuntimeCC());
 
       if (!Local && getTriple().isOSBinFormatCOFF() &&
-          !getCodeGenOpts().LTOVisibilityPublicStd) {
+          !getCodeGenOpts().LTOVisibilityPublicStd &&
+          !getTriple().isWindowsGNUEnvironment()) {
         const FunctionDecl *FD = GetRuntimeFunctionDecl(Context, Name);
         if (!FD || FD->hasAttr<DLLImportAttr>()) {
           F->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
           F->setLinkage(llvm::GlobalValue::ExternalLinkage);
         }
       }
     }
   }

cfe/trunk/test/CodeGenCXX/runtime-dllstorage.cpp

 // RUN: %clang_cc1 -triple i686-windows-msvc -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-MS -check-prefix CHECK-MS-DYNAMIC
 // RUN: %clang_cc1 -triple i686-windows-msvc -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -flto-visibility-public-std -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-MS -check-prefix CHECK-MS-STATIC
 
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-DYNAMIC-IA -check-prefix CHECK-DYNAMIC-NODECL-IA -check-prefix CHECK-DYANMIC-IA-CXA-ATEXIT
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -flto-visibility-public-std -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA -check-prefix CHECK-STATIC-NODECL-IA -check-prefix CHECK-IA-STATIC-CXA-ATEXIT
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -DIMPORT_DECLARATIONS -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-DYNAMIC-IA -check-prefix CHECK-DYNAMIC-IMPORT-IA -check-prefix CHECK-DYANMIC-IA-CXA-ATEXIT
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -flto-visibility-public-std -DIMPORT_DECLARATIONS -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA -check-prefix CHECK-STATIC-IMPORT-IA -check-prefix CHECK-IA-STATIC-CXA-ATEXIT
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -DEXPORT_DECLARATIONS -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-DYNAMIC-IA -check-prefix CHECK-DYANMIC-IA-CXA-ATEXIT
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -flto-visibility-public-std -DEXPORT_DECLARATIONS -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA -check-prefix CHECK-IA-STATIC-CXA-ATEXIT
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -DDECL -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-DYNAMIC-IA -check-prefix CHECK-DYNAMIC-DECL-IA -check-prefix CHECK-DYANMIC-IA-CXA-ATEXIT
 // RUN: %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -flto-visibility-public-std -DDECL -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA -check-prefix CHECK-STATIC-DECL-IA -check-prefix CHECK-IA-STATIC-CXA-ATEXIT
 // %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -fno-use-cxa-atexit -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-DYNAMIC-IA -check-prefix CHECK-DYNAMIC-IA-ATEXIT
 // %clang_cc1 -triple i686-windows-itanium -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -fno-use-cxa-atexit -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA -check-prefix CHECK-STATIC-IA-ATEXIT
 
-// RUN: %clang_cc1 -triple i686-windows-gnu -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-DYNAMIC-IA
+// RUN: %clang_cc1 -triple i686-windows-gnu -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA
 // RUN: %clang_cc1 -triple i686-windows-gnu -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -flto-visibility-public-std -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA
 // RUN: %clang_cc1 -triple i686-windows-cygnus -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-DYNAMIC-IA
 // RUN: %clang_cc1 -triple i686-windows-cygnus -std=c++11 -fdeclspec -fms-compatibility -fexceptions -fcxx-exceptions -flto-visibility-public-std -emit-llvm -o - %s | FileCheck %s -check-prefix CHECK-IA -check-prefix CHECK-STATIC-IA
 
 #if defined(IMPORT_DECLARATIONS)
 namespace __cxxabiv1 {
 extern "C" {
 __declspec(dllimport) void __cxa_guard_acquire(unsigned long long *);